Field of the Invention
Embodiments of the invention relate to the field of substrate manufacturing. More particularly, the present invention relates to a method, system and structure for growing a crystal sheet from a melt.
Discussion of Related Art
Silicon wafers or sheets may be used in, for example, the integrated circuit or solar cell industry. Demand for solar cells continues to increase as the demand for renewable energy sources increases. One major cost in the solar cell industry is the wafer or sheet used to make these solar cells. Reductions in cost to the wafers or sheets will, consequently, reduce the cost of solar cells and potentially make this renewable energy technology more prevalent.
One method that has been investigated to lower the cost of materials for solar cells is the vertical pulling of thin silicon ribbons from a melt that cool and solidify into a crystalline sheet. The pull rate of this method may be limited to less than approximately 18 mm/minute. The removed latent heat during cooling and solidifying of the silicon must be removed along the vertical ribbon. This results in a large temperature gradient along the ribbon. This temperature gradient stresses the crystalline silicon ribbon and may result in poor quality multi-grain silicon. The width and thickness of the ribbon also may be limited due to this temperature gradient.
Producing sheets (or “ribbons”) horizontally from a melt, such as so-called horizontal ribbon growth (HRG) has also been investigated. Earlier attempts employed helium convective gas cooling to achieve the continuous surface growth needed for ribbon pulling. These early attempts have not met the goal of producing a reliable and rapidly drawn wide ribbon with uniform thickness that is “production worthy.”
Although radiative cooling of a silicon melt has been proposed as an alternative method of forming crystalline silicon, a problem arises because the large difference in emissivity ϵs−ϵl between solid and liquid silicon makes it difficult to obtain rapid solidification of the melt surface using radiative cooling. In view of the above, it will be appreciated that there is a need for an improved apparatus and method to produce horizontally grown silicon sheets from a melt.